The message notifier box will normally have "No Msg" in black text on a white background if you have no messages waiting for you.
If you have received a message from another station, the background of the message notifier box will turn green, and the callsign of the sending station will be displayed in the box. Double clicking the green box will show you the first unread message.
If you have sent a message that has not yet been acknowledged, this box will be yellow. Selecting Messages / Pending Messages (or double-clicking the yellow box) will show you what message(s) are still pending.
If you have both unread (inbound) and unacknowledged (outbound) messages, this box will be orange and double-clicking it will give priority to the incoming message.